Optical holographic materials can be used for high-density data storage and high-speed data processing in both digital and analog forms. Information is stored in a holographic material in form of holograms. An optical signal beam can be modulated to carry the information to be stored or processed. A reference beam, which is coherent with the signal beam and provides a substantially homogeneous illumination, overlaps with the signal beam in the holographic material to produce an optical interference pattern. The holographic material responds to the interference pattern and produces a hologram as a spatial variation in the index of refraction that replicates the interference pattern. To retrieve the information, a readout beam is used to illuminate the holographic material and is diffracted by the recorded hologram to produce a reconstruction beam. The reconstruction beam is then detected to retrieve the recorded information.
Photorefractive materials are one class of widely-used holographic materials. A photorefractive material has spatially-distributed photorefractive centers capable of producing charge carriers (e.g., electrons) by absorbing photons. The inhomogeneous illumination caused by the interference pattern of the signal and reference beams excites charge carriers from the photorefractive centers. The charge carriers migrate and become trapped by other photorefractive centers at different locations. Such charge migration and trapping produce a space-charge field that replicates the interference pattern. Due to the electro-optic effect, a spatial variation in the index of refraction is generated to form the hologram. Low optical powers can be used to record holograms through the above photorefractive effect and a high percentage of a readout beam can be diffracted into the reconstruction beam to achieve a high diffraction efficiency. Hence, photorefractive materials have been used in many holographic devices including optical storage devices.
The above photorefractive process in many photorefractive materials is reversible and thus can be used to implement rewritable holographic storage. Since the readout beam is a spatially-homogeneous beam, the charge carriers can be redistributed by the readout beam. Hence, the readout process also erases the recorded hologram. However, such volatile readout is undesirable in applications where the recorded information needs to be repetitively accessed.